1. Field of the Invention
The present invention relates to a surface acoustic wave device which is used, for example, as a resonator or a band pass filter, and more particularly, to a surface acoustic wave device in which an IDT is formed by using a metal filled in grooves on a piezoelectric substrate.
2. Description of the Related Art
Heretofore, as a resonator and/or a band pass filter, a surface acoustic wave device has been widely used. For example, in WO2006/011417A1, a surface acoustic wave device 1001 having a cross-sectional structure schematically shown in FIG. 26 has been disclosed.
In the surface acoustic wave device 1001, a plurality of grooves 1002b are formed in an upper surface 1002a of an LiTaO3 substrate 1002. A metal is filled in the plurality of grooves 1002b, and thereby an IDT 1003 including a plurality of electrode fingers made of the above metal is formed. An SiO2 film 1004 is laminated so as to cover the upper surface 1002a of the LiTaO3 substrate 1002. Since the LiTaO3 substrate 1002 has a negative temperature coefficient of frequency TCF, the SiO2 film 1004 having a positive temperature coefficient of frequency TCF is laminated to the LiTaO3 substrate 1002, so that the absolute value of the temperature coefficient of frequency TCF of the surface acoustic wave device 1001 can be decreased.
In addition, it is believed that since the IDT is formed by using the metal disposed in the plurality of grooves 1002b, a high reflection coefficient is obtained in the IDT. In particular, when the wavelength of a surface acoustic wave is represented by λ, and the thickness of Al filled in the grooves 1002b, that is, the thickness of the IDT made of Al, is set to 0.04λ, the reflection coefficient per one electrode finger is 0.05, and it has been shown that as the thickness of the electrode is increased, a higher reflection coefficient can be obtained.
In addition, in Japanese Unexamined Patent Application Publication No. 2004-112748, a surface acoustic wave device shown in FIG. 27 is disclosed. In a surface acoustic wave device 1101, an IDT 1103 is provided on a piezoelectric substrate 1102 made of LiTaO3 or LiNbO3. In addition, a protective film 1104 is arranged so as to cover the IDT 1103. On the other hand, in a region other than that in which the IDT 1103 and the protective film 1104 are provided, a first insulating layer 1105 made of SiO2 is provided which has a thickness equal or substantially equal to that of a laminated metal film of the IDT 1103 and the protective film 1104 laminated to each other. In addition, a second insulating layer 1106 made of SiO2 is laminated so as to cover the first insulating layer 1105. In this case, it has been shown that since a metal having a density greater than that of Al is used for the IDT 1103, the absolute value of the reflection coefficient can be increased, and undesired ripples can be suppressed.
In the surface acoustic wave device 1001 disclosed in WO2006/011417A1, it has been shown that as the thickness of the IDT made of Al is increased, the absolute value of the reflection coefficient can be increased. However, the inventors of the present invention discovered that when the absolute value of the reflection coefficient is simply increased, superior resonant characteristics cannot be obtained. That is, in the surface acoustic wave device disclosed in WO2006/011417A1, although the absolute value of the reflection coefficient can be increased by increasing the thickness of the electrode made of Al, it was found that since the sign of the reflection coefficient is negative, many ripples are generated in a pass band, and thus, superior resonant characteristics cannot be obtained.
In WO2006/011417A1, for the relationship between the thickness of the IDT and the reflection coefficient, only the case in which an IDT made of Al is used on an LiTaO3 substrate has been described. In addition, in paragraph 0129 of WO2006/011417A1, it has been suggested that the IDT may be formed using another metal, such as Au. However, only an IDT made of Au has been disclosed.
In addition, according to Japanese Unexamined Patent Application Publication No. 2004-112748, it has been disclosed as described above that when the IDT made of a metal having a density higher than that of Al is used, the absolute value of the reflection coefficient can be increased. However, an increase in electromechanical coupling coefficient of a surface acoustic wave device to be obtained has not been particularly described.
In addition, in the structure in which the IDT is formed by filling Au in the grooves provided in the above LiNbO3 substrate, there has been a problem in that the range of the Euler angles of the LiNbO3 substrate which can be used to obtain a sufficiently high electromechanical coupling coefficient K2 is very small.